Field of the Invention
The present invention relates generally to thin films exhibiting metamaterial properties in their as-grown/deposited form, that is films that due to the naturally occurring structure within the film exhibit properties not inherent to the material and, more specifically, to thin film materials deposited or grown by atomic layer deposition or related methods that exhibit plasmonic properties (sub-diffraction limit confinement of light).
Description of the Prior Art
Surface enhanced Raman scattering (SERS) is a technique in which the Raman signal can be enhanced by many orders of magnitude by the use of metal nanoparticles. This enhancement is due to the local electromagnetic fields that are created by the optical excitation of localized surface plasmons. The size, geometry, shape, and alignment of the metallic nanoparticles are important parameters for this enhancement. The metal that exhibits the highest SERS in the visible part of the spectrum is silver, and many SERS enhancement studies have been reported for Ag-based nanoparticles.
Common SERS substrates are based on roughened Ag surfaces produced by etching or chemical deposition, metal coated nanostructures or structures that are lithographically fabricated. The rough substrates work because the roughness is spherical in nature with a Gaussian distribution of sizes. Some disadvantages of these roughened Ag surfaces are that there is no control over plasmonic properties, there is a random nature of roughness, and it cannot be deposited on many substrates including insulators, fabrics, or large areas. Alternative approaches typically suffer from random structures, leading to non-uniform SERS response, or from the high expense associated with lithographic fabrication efforts.
Spoof and hybrid-spoof plasmonics emerge from the properties of metallic conductors with surface geometries defined to mimic the dispersion and confinement of a surface plasmon polariton (SPP) and the confinement of photonic fields in subwavelength regions. In these systems, the operating frequency of the spoof plasmon is defined primarily by the geometry and thus can be used to create designer plasmonic devices. Spoof plasmonics can be achieved with any conductor at frequencies below the plasma frequency. In addition to metals, spoof plasmonics can also be created from highly doped semiconductors or polar dielectrics. A special case of spoof plasmonics is hybrid-spoof plasmonics which uses traditional plasmonic materials such as Ag, Au, Cu, Pt and/or Al within the frequency range at which they can support SPPs along with the fabrication of geometric patterns lithographically defined to induce spoof-plasmon excitations to enable the further control of the SPPs and/or multiple resonant structures. Both spoof and hybrid-spoof plasmon approaches are ideal to create new metamaterials, which have properties normally not found in nature in the near- to mid-IR. The disadvantage of this approach is that lithographic patterns must be formed in order to define the hybrid spoof geometry, and more importantly, if one needs to access hybrid spoof plasmonics in the visible part of the spectrum, the required lithography is too difficult to obtain even by e-beam lithography, due to the small size scales involved (features require spacing on the order of 1-20 nm in separation).